To examine how people deal with perceivable consequences of their voluntary actions, we recorded event-related potentials (ERPs) during a self-paced, two-choice random generation task. Sixteen participants were asked to press one of two buttons randomly at a regular but self-selected interval of once per 1–2 s. Each button press produced either a 1000-Hz or 2000-Hz tone, but participants were told that the tones were irrelevant to the task. The button–tone combinations were initially fixed, but in subsequent blocks, a button press infrequently produced the tone associated with the opposite button (p=.15). This cognitively mismatched tone elicited N2, P3, and late positive potential (or positive slow wave) of the ERP and delayed the timing of the next button press. These results suggest that action effects are difficult to ignore and that an action effect that is different from a performer's expectation may cause task disruption.